I know there is IHL field in IPV4 which is Internet Header Length.Since an IPv4 header may contain a variable number of options, this field specifies the size of the header. But there is no IHL or something like that in IPV6.So how to get the exact size of header length?
(We know There may be extensions.I need to know the size of IPv6 header including header extensions but without upper layer data.)

It's difficult to tell what is being asked here. This question is ambiguous, vague, incomplete, overly broad, or rhetorical and cannot be reasonably answered in its current form. For help clarifying this question so that it can be reopened, visit the help center.
If this question can be reworded to fit the rules in the help center, please edit the question.

Why do you need to know this? Your question is very abstract, you might get a more useful answer if you can explain what you are really after.
–
ZoredacheApr 30 '13 at 17:32

Actually you don't need to check it because IPV6 have a fixed header lengh:

The IPv6 packet header has a fixed size (40 octets). Options are
implemented as additional extension headers after the IPv6 header,
which limits their size only by the size of an entire packet. The
extension header mechanism makes the protocol extensible in that it
allows future services for quality of service, security, mobility, and
others to be added without redesign of the basic protocol.

The fixed header occupies the first 40 octets (320 bits) of the IPv6
packet. It contains the source and destination addresses, traffic
classification options, a hop counter, and the type of the optional
extension or payload which follows the header. This Next Header field
tells the receiver how to interpret the data which follows the header.
If the packet contains options, this field contains the option type of
the next option. The "Next Header" field of the last option, points to
the upper-layer protocol that is carried in the packet's payload.

This was a choice of IPV6 comitee to increase performance(in comparison with IPV4), you don't need to check the size on a parameter inside the header having a fixed number of bytes to check for every packet.